A double-blind, placebo controlled study of the effectiveness of a monoclocal antibody (493) to respiratory syncytial virus(RSV)in preventing hospitalization for bronchiolitis/pneumonia in infants at high risk, born prematurely and/or with a diagnosis of bronchopulmonary dysplasia (BPD).Patients will receive either the study drug(MED-493)or placebo by intramuscular injection once per month during the peak season for RSV infection (November-April). Blood will be drawn for toxicity monitoring.